Since silicate builders have alkaline buffering properties, they are highly useful for detergent builders. Particularly, since the silicate builders in crystalline forms have excellent cationic exchange capacities, they are greatly useful for detergent builders.
Among methods for producing the silicate builders, Japanese Patent Laid-Open No. 5-66888 discloses a method using water glass as a starting material. In this method, a crystalline sodium silicate having a layered structure is produced by a method comprising the steps of spray-drying a water glass solution to form amorphous sodium silicate, and heating the resulting amorphous sodium silicate in a firing zone while recirculating a portion of the product. However, this method requires drying off large amounts of the water content from the water glass, which makes it disadvantageous from an energy-saving viewpoint.
Japanese Patent Laid-Open No. 6-72008 discloses a method for directly baking the glassy product to solve the above problems. In this method, crystalline sodium silicate is produced by the steps of pulverizing a water glass product obtainable by cooling a fused mixture of sand and soda, and then heating the pulverized product. However, in this method, since the resulting product comprises a two-component system of Na.sub.2 O--SiO.sub.2, the control of the crystalline system becomes difficult, rendering its performance as an ion exchange material for detergents unsatisfactory. Also, a large amount of heat energy is consumed upon glassification, thereby making it disadvantageous from an energy-saving viewpoint.
In view of the above problems, an object of the present invention is to provide a method for easily producing a high-performance silicate builder using inexpensive silica sand as a starting material.
These and other objects of the present invention will be apparent from the following description.